The evolution to the high speed and high quality wireless packet data communication system has transformed the voice-oriented communication service to the data and multimedia services. The High Speed Packet Access (HSPA) and Long Term Evolution (LTE) of the 3rd Generation Partnership Project (3GPP), High Rate Packet Data (HRPD) and Ultra Mobile Broadband (UMB), and 802.16e of the Institution of Electrical and Electronics Engineers (IEEE) are the mobile communication standards developed for supporting such high speed and high quality wireless data transmission services.
These recent mobile communication technologies use sophisticated techniques such as an Adaptive Modulation and Coding (AMC) and/or channel-sensitive scheduling. The AMC is a technique to adjust the transmission amount of data according to the channel conditions. This means that the transmitter decreases the transmission data amount in bad channel condition to limit the received signal error probability to a wanted level and increases the transmission data amount in good channel condition while maintaining the signal error probability level. The channel sensitive scheduling allows the transmitter to allocate channels to multiple users selectively according to the channel condition so as to increase the system throughput. In the system using the AMC and channel-sensitive scheduling, the transmitter receives feedback from the receivers and performs transmission at the most effective timing with most appropriated modulation and coding scheme.
Meanwhile, the wireless access technology which is dominant in 2G and 3G system is evolving from Code Division Multiple Access (CDMA) to Orthogonal Frequency Division Multiple Access (OFDMA). The 3GPP and 3GPP2 have begun working on the standardization of the OFDMA-based evolved system. OFDMA is known superior to CDMA in capacity improvement.
It is one of the various factors for increasing the system capacity that the OFDMA can exploit frequency domain scheduling. In addition to the channel-sensitive scheduling, the frequency domain scheduling allows further system capacity gain in time-varying channel environments.
The AMC and channel-sensitive scheduling are technologies to improve the transmission efficiency based on the information about the transmission channels. Typically, a Frequency Division Duplex (FDD) system, in which the transmitter cannot infer the condition of its transmission channel, is designed such that the receiver reports the channel information to the transmitter. However, since the channel condition varies as time progresses in the wireless communication environment, feedback delay causes degradation of the efficiency of the AMC and channel-sensitive scheduling. This can be worse when the receiver is in high mobility state. Accordingly, there is a need for a supplementary transmission scheme to maintain the ongoing communication at least reliable level even when the channel condition feedback has become unreliable.
The technologies that are typically referred to as diversity methods are known less sensitive to the channel conditions. For instance, a frequency diversity method performs transmission through the frequency channels spaced enough on the frequency domain. Considering the frequency selective fading environment, the responses of the channels spaced enough are less correlated with each other. This means that the frequency diversity transmission can reduce the probability of any worst case of transmission failure because, even when one frequency resource experiences bad channel condition, the other frequency resource does not do.
Spatial diversity is another diversity scheme that uses multiple transmission and/or reception antennas. Assuming the transmission and/or reception antennas are spaced far enough from each other, the channel responses of the antennas are less correlated. Accordingly, the spatial diversity can decrease the probability to experience of the worst transmission condition even when one of the antennas experiences a bad channel response.
Transmission diversity is a special case of the spatial diversity applied to the transmitter. There are various transmission diversity techniques including Selective Transmission diversity, Space Time coding, Orthogonal Transmission diversity, etc.
In a cellular communication system, a base station serves mobile stations within its radio coverage, also referred to as cell, and triggers a handover of the mobile station moving out of its coverage to a neighbor base station for maintaining the ongoing call. In the cellular structure, the user located at the boundary of a cell is likely to experience the interference caused by the signal of neighbor base stations, resulting in bad channel state. Also, the closer the mobile station is to the base station, the higher the service transmission rate is.
In order to solve this problem, the 4th Generation (4G) mobile communication systems are expected to be implemented with a newly introduced technique called collaborative transmission in which the adjacent base stations transmit the same signal to a mobile station located at the cell boundary.
The collaborative transmission techniques can be classified into low level collaborative transmission technique and high level collaborative transmission technique. In the low level collaborative transmission technique, the base stations do not share the signal transmission but collaborates to make scheduling and beamforming decisions. In contrast, the high level collaborative transmission technique, the base stations collaborate for actual signal transmission as well as the scheduling and beamforming decisions. Although it increases the network traffic due to the increase of information exchanged between the base stations, the high level collaborative transmission technique is advantageous to improve the channel condition at the cell boundary since the mobile station can achieve transmission diversity gain from the transmissions of the base stations.
The transmission diversity is known to be optimized with two transmission antennas, since the recipient device can achieve the coherent combination of the received signals using the orthogonality without compromising data rate. In case that two cells, each having a single transmission antenna, are involved, it is ease to implement the high level collaborative transmission in which the both the base station transmit the same signal, resulting in transmission diversity gain. In other cases using more than two transmission antennas, however, other diversity technique, rather than the optimized transmission diversity, has to be adopted.
For instance, when three cells, each having a single transmission antenna, are involved in the collaborative transmission, another transmission diversity technique is required to achieve transmission diversity gain. Also, when two bases stations, each having multiple transmit antennas, are involved in the collaborative transmission, still another diversity technique is required.